Kim Beazley's decision to
proceed with the acquisition of Airborne Early Warning aircraft for the
RAAF is one which informed observers will all agree to be long overdue.
This journal argued strongly for the acquisition of AEW aircraft a half
a decade ago and needless to say none of the issues under examination
at
the time have changed. While it is nice to see one's assertions
vindicated the issue is now one, of which system is best suited to the
RAAF's unique operational environment.

The RAAF is facing a non-trivial decision. Modern Airborne
Warning and Control/Airborne Early Warning/Airborne Surveillance
Warning
and Control (AWACS/AEW/ASWAC-the reader is free to choose, we will use
the appelation AEW without implying a specific design) aircraft are
complex systems integrating a large look-down radar, secondary radar
(IFF), Electronic Support Measures (ESM) and a broad array of voice and
data communication equipment, all of these carefully traded off to
support a specific air defence environment.

AEW emerged in the immediate post war period when the US
Navy,
responding to the painfully learned lessons of defending against the
kamikaze, deployed its TBM-3W Avengers. The idea of carrying a search
radar on an aircraft stemmed from a fundamental geometrical limitation
of surface based radar, its inability to see over the horizon. From a
warship, subject to the height of the antenna, the radar horizon is of
the order of 15nm away or less with diminishing antenna height. This
constraint and the less demanding ocean clutter environment made for a
large payoff in the use of AEW at sea, putting the radar antenna onto
an
aircraft being the simplest way of extending the radar horizon. The USN
and RN adopted AEW very promptly and deployed AN/APS-20 radar equipped
AD-3W, AD-4W and AEW.1 Skyraiders on several carriers, while the USN
and
USAF fitted the radar to long range land based WV-1/2 and EC/RC-121 C/D
derivatives of the Lockheed Constellation.

These APS-20 based systems represent the first generation of
AEW systems and saw extensive use well into the seventies, when the
USAF
deployed its C-137(707) based E-3A and the USN phased out the last of
its land based AEW in favour of carrier based E-2B/C Hawkeyes. Carrier
based AEW has had a colourful history with the USN deploying the AD-3W,
AD-4W, E-IB Tracer and finally the very successful E-2B/C family, while
the RN transplanted its APS-20s into the Gannet AEW and subsequently
passed these radar sets to the RAF where they equipped the long serving
Shackleton AEW.

At this instant in time, AEW has been widely recognised as a
key part of a modern air defence system, with the E-2C deployed by the
USN, Japan, Israel, Egypt, Singapore and sought by Pakistan, the
E-3A/B/C deployed by the USAF, NATO, Saudi Arabia and on order with the
RAF and Armee de I'Air, the P-3 AEW&C deployed by US Customs, the
II-78 based Mainstay deployed by Russia's IA PVO and the Madcap under
development.

Proposals involving 707, C-130 and other airframes seem to be
very common in the late eighties.

Boeing
E-3A AWACS (Boeing
images).

AEW - A System Perspective

The intensity and pace of the modern air battle with its
emphasis upon the disruption of Command, Control and Communications (C)
facilities with electronic warfare techniques and exploitation of low
level penetration to targets, dictates the use of AEW if a successful
air defence umbrella is to be maintained. The ability of an AEW
platform's look-down radar to strip away the sanctuary of terrain
following flight, removes a key advantage possessed by the attacker,
surprise. To fully exploit the high ground held by the AEW platform,
it is however, essential that secondary radar (Identification Friend
Foe
(IFF)) be carried to manage own aircraft and extensive voice and data
communication systems be installed to support friendly aircraft and
surface based defences. To this must be added ESM, sophisticated radar
warning receivers, which allow passive long range detection of inbound
threats and provide the ability to identify non-cooperating tracks (ie
those which ignore IFF).

With several hundred tracks generated by the radar, IFF and
ESM, the task of correlating these tracks and maintaining track
histories, demands automation and hence powerful computers must be
carried. These will interface to the operators via sophisticated
consoles, while considerable mass storage will be required to support
operation and log track histories. This hardware alone is complex and
demanding of volume, payload, cooling capacity and power, all of which
in turn, place demands upon the airframe and powerplants. To this
complexity however,must be added the software, some of which carries
out
specialised tasks such as signal processing and most of which controls
the radar, IFF, ESM and C hardware while driving operator consoles and
maintaining track histories.

AEW is therefore expensive to design, build, deploy, support
and train aircrew for, this is the price an air force must pay for a
priceless tactical advantage. Airborne Pulse Radar and Clutter

The radar system of an AEW platform is the key to much of its
capability and in the final analysis, a measure of its worth
tactically.
The look-down performance of such a radar at long ranges over land
mass,
is usually seen as the determining performance criterion when judging
an
AEW platform, weaknesses in this area led to the much publicised demise
of the RAF's BAe/GEC Nimrod AEW.3 aircraft three years ago. Modern AEW
aircraft carry two forms of radar, Moving Target Indicator (MTI) and
Pulse Doppler (PD), both of which are essentially pulse radars with
additional signal processing to exploit the Doppler effect in locating
targets otherwise hidden by surface clutter.

Lockheed AMSS proposal. Lockheed's Advanced
Multimission
Sensor System proposal for a Carrier based AEW aircraft is interesting
as it employs a fixed three sided dorsal radome covering what would be
three phased array antennas. Phased arrays are built up of thousands of
miniature receive / transmit elements which allow nearly instantaneous
electronically controlled pointing of very low sidelobe beams, with
good
frequency agility. This will provide high resistance to jamming while
also defeating most Anti-Radiation Missile seekers which home in on a
radar's sidelobe transmissions.

Radar in the general case involves the detection of targets
by
illuminating them with an electromagnetic wave and then listening for
the reflection (echo) of this wave from the target. Because
electromagnetic waves travel at a constant speed (of light), numerous
techniques can be used to then determine the range of the target (and
hence Radio Detection And Ranging). A radar always has three basic
components, a transmitter chain, an antenna and a receiver chain. The
transmitter chain is a high power source of electromagnetic signal and
it can be built as a high power oscillator (typically pulsed Magnetron)
or as a MOPA (Master Oscillator Power Amplifier) chain using typically
Travelling Wave Tubes. The waves it produces must be focussed into a
beam and pointed in some sought of direction, this is one of the
functions of the antenna. The beam then propagates through space, hits
a
target and some of the power in the beam is scattered back in the
direction of the illuminating radar antenna.

Because the wave weakens with distance covered (inverse
square
law) and the target reflects only a small fraction of the impinging
wave, the return (echo) will be very faint by the time it arrives back
at the radar antenna.

Here is where the antenna comes into play again, focussing
and
concentrating the faint return at the input of a sensitive receiver
termed a front end.

The received return signal is then manipulated in a number of
clever ways, amplified substantially and in the simplest case fed to a
Cathode Ray Tube which displays an image synchronised to the motion of
the antenna.

The observant reader will note that this notional radar is
transmitting while receiving through a single antenna, in practice this
is seldom done as the amount of power leaking from the transmitter feed
into the receiver feed would blind the receiver to the weak echoes it
is
listening for.

Real surveillance radars are built as Pulse radars. A pulse
radar transmits only for a very brief time, a short burst (pulse)
usually about a microsecond or so in duration at a time, after which it
listens for echoes as the pulse propagates away.

Because the Pulse is receding away from the radar at a
constant speed, the time elapsed from the instant the pulse was sent to
the time an echo is received, is a clear measure of the line-of-sight
distance to the target. The next pulse can only be sent after a time
which is equal to the round trip delay of a pulse reflecting from a
target at the limit of the radar's detection range (given by
transmitter
power, antenna gain and receiver sensitivity). If a pulse were sent any
earlier, the echo of the first (ie; preceding) pulse from a distant
target could be confused with an echo of the second pulse from a close
target (the reader is advised to sketch this).

The time gap between pulses is termed Pulse Repetition
Interval (PRI), its inverse is an important parameter termed Pulse
Repetition Frequency (PRF). Low band long range surveillance radars
have
PRFs typically of hundreds of pulses per second (or Hertz [Hz]).
Needless to say PRF is a distinctive signature betraying the identity
and operating mode of a radar to any eavesdropping RWR.

This generic pulse radar is typical of early systems and
performs only when the background beneath the target is clear airspace.

If such a radar looks at a target flying above eg; a large
hill, the return from the hill will be far stronger than the return
from
the aircraft and the target will be obscured. The term used for these
unwanted echoes from Mother Nature's splendour is clutter. Needless to
say clutter was a cause of great pain to early radar designers and has
only been dealt with adequately in the last two decades. Airborne pulse
radars are particularly vulnerable to clutter which most effectively
hides low flying targets, these radars are usable only against targets
at the same or at a higher altitude.

Techniques for dealing with clutter exploit in one or another
way, the Doppler effect (the frequency of a wave reflected off an
approaching object is increased, a receding object decreased). The
simplest radar which can detect an object in clutter is a Moving Target
Indicator radar. An MTI is a pulse radar which will compare the returns
over two or more pulse repetition intervals. Any target which is,
moving
relative to the radar will register a change in a signal parameter
(phase in a coherent MTI), whereas the background clutter is unchanged.
Clutter cancellation is then carried out by subtracting the returns
from
the two successive intervals. In practice cancellation can be
accomplished in hardware (delay line cancellers or analogue bucket
brigade chips) or by algorithms in software.

MTIs operating at a constant PRF suffer a fundamental
weakness, they are blind to targets with particular relative velocities
(which incur phase shifts of multiples of 180 degrees) and are said to
be ambiguous in velocity. Modern MTIs transmit multiple staggered PRFs
such that the blind speeds at each PRF are covered by other PRFs.

An alternative family of techniques for dealing with clutter
fall under the designation of Pulse Doppler radar. PD radars employ
substantially more complex processing than MTIs. In a PD radar the
returns containing targets and clutter are fed into a bank of Doppler
filters, each of which is tuned to a particular frequency ( - Doppler
-> speed). In this fashion targets with given speeds register
as outputs from given filters, which may be implemented in hardware or
software. In this basic form, a PD radar cannot resolve target range
(of
MTI) and is said to be ambiguous in range. Further manipulation is thus
required to define range, this is usually done by dividing the pulse
repetition interval into slices termed range cells, the Doppler filter
bank is then selectively fed during a given range cell. This is termed
range gating and results in a set of filter outputs indicating the
velocity of any targets detected in that range cell. Because typical
Pulse Doppler radars operate at medium to high PRFs (usually thousands
of Hertz) the above technique becomes ambiguous in range and multiple
staggered PRFs are used (of MTI) together with some clever processing
to
decide which actual range the target is at (it will appear in different
range cells at different PRFs).

Pulse Doppler radars are usually considered superior to MTI
in
the detection of low flying targets in heavy ground clutter, it is the
preferred technique used in modern fighter air intercept/fire control
radars eg the AN/APG-63, 65, 66, 67 and 70. In current PD radar most
processing is carried out by a dedicated digital computer termed a
digital signal processor, usually executing a powerful algorithm termed
a Fast Fourier Transform to convert the digitised stream of return
samples into something more manageable by other algorithms. DSPs are
very flexible in that the algorithms used can usually be rapidly
changed
by swapping read only memory (ROM) chips, thus rapidly countering an
opponent's jamming technique if necessary.

Airborne Doppler Radar

The carriage of radar on a aircraft introduces a wide range
of
problems. The motion of the aircraft dictates that the antenna be
stabilised in pitch, roll and yaw, to provide a stable jitter-free
picture, this becomes a major effort with increasing antenna size and
weight. The other major problem area is of course clutter. A moving
aircraft will itself have a Doppler relative to the Earth's
surface, hence shifting the nett clutter return in frequency.

At first glance this would be a simple problem to tackle, by
filtering out all of the return with the expected ground Doppler ,
which is given by the aircraft's instantaneous ground speed and the
direction the antenna is pointed in. Reality isn't so simple though, as
antennas are not ideal and will at best try to produce a tight beam,
spilling out power in weaker off-axis beams termed sidelobes (and
receiving unwanted weaker returns from the direction of these beams).
As
is apparent the different directions the sidelobes point in, will
result
in different Doppler shifts as compared to the antenna mainlobe, with
the result of course that the clutter produced by the sidelobes will be
difficult to remove.

This is further complicated by the unfortunate reality that
the aircraft's airframe itself, interacts with the mainlobe and
sidelobes, reflecting these off in directions very different to the
axis
of the antenna. Needless to say, this further complicates the rejection
of clutter and imposes the need for very careful antenna placement on
the airframe.

This problem is exacerbated where the radar operating
wavelength is large, relative to the antenna size, as the sidelobes are
larger (in general the larger the antenna relative to the wavelength,
the tighter the beam). It is for this reason that PD radars designed
specifically for overland look-down performance, use shorter
wavelengths
(typically E/F band for AEW with 3-8m antennas and I/J band for Air
Intercept with 1 m or smaller antennas), even if this incurs a major
cost penalty due to more expensive (faster) transmitter chains and
receivers (faster, more sensitive).

This aspect of performance can be tricky to assess, in that
longer wavelengths do tend to provide stronger and more stable returns
(resonance region radar cross section, see TE May 1987) with better
skin
depth penetration of radar absorbent (stealthy) skins, as compared to
shorter (microwave) wavelengths where returns scintillate (optics
region
RCS) with target aspect. The now you see it, now you dont' aspect of
a
scintillating target imposes the need for a capable data processing
system with sophisticated tracking algorithms (Kalman filtering
techniques are commonly used).

The decision to use a given wavelength and antenna
configuration is thus not trivial, with substantial cost and system
complexity penalties attached to the use of higher performance
microwave
band antennas.All of these factors impact upon the physical
installation
within the aircraft. More sophisticated electronics tend to consume
more
power, require more cooling and occupy more space, while costing more
and requiring more maintenance by better qualified personnel.

While discussing performance tradeoffs, resistance to hostile
jamming ie ECCM performance must be considered, particularly where one
is up against a clever opponent. Antennas with large siddelobes are
inherently vulnerable to high power noise jammers and false target
generators, while a modern AEW radar must have a measure of frequency
agility, the ability to quickly tune away from a jammer, preferably
automatically. ECM is however, a non lethal threat which can be fought
with clever ECCM, the proliferation of high performance Anti-Radiation
Missiles will eventually make life quite difficult for conventional AEW
platforms. The only real counter to these, is the use of sophisticated
Low Probability of Intercept (LPI) techniques (pseudorandom signal
formats and scan patterns) requiring phased array antennas and very
sophisticated signal processing, scan control and data processing. This
will result in further complexity in hardware and particularly
software,
but with a major pay off in resistance to jamming and an ability to
disrupt tracking by an ARM seeker.

IFF, ESM, Data Processing and
C3 Systems

Given that a radar of such capability is successfully fitted
to an airframe, it must be integrated with a suitable data processing
system, software and operator consoles. The software running on the
computer(s) at the core of this system, will command the radar and its
dedicated signal processor to given operating modes, while receiving
and
managing target track information produced by the radar. Tracks are
then
displayed on operator consoles.

Target tracks are usually identified or tagged, depending on
their IFF and ESM status, a task termed association which involves
matching up radar target tracks with IFF tracks and ESM bearing/type
readings. IFF, which relies on coded responses by transponders carried
by friendly aircraft, is an essential comple ment to an AEW radar,
providing positive identification of responding friendly aircraft. IFF
interrogator antennas are usually mounted back to back with radar
antennas in rotodomes or on axis with conventional (eg offset parabolic
antennas), sharing access ducts and implicitly antenna motion
stabilisation.

Electronic Support Measures are an often overlooked
complement
to an AEW platform's primary and secondary radar, but are becoming
increasingly important in a modern air battle. ESM equipment comprises
sensitive direction finding radar warn ing receivers, coupled to an
extensive software threat library and able to produce bearing and type
tracks in a format readable by the data processing software, passively
identifying the source of a transmission at ranges of the order of
twice
that of radar with comparable receive sensitivity. Where a target is
not
responding to IFF interrogation and is therefore potentially hostile,
ESM identification can provide confirmation of, or disprove its status
hence avoiding tragedies such as the Persian Gulf Aegis incident, this
unfortunate event provides a clear illustration of the pitfalls
implicit
in air defence operations where decisions are based on the outputs from
a single sensor, eg radar.

Complete target identity information is today unattainable,
given the state of the art in sensors, but well implemented
radar/IFF/ESM integration can go a long way in resolving uncertainties.
Association of radar, IFF and ESM tracks can take place prior to the
radar track information being passed to data processing or be done in
software by the data processing system, which will typically create a
track file on mass storage (disc or drum in older machines), the file
containing target parameters and being maintained as long as the target
is tracked. It is worth noting, that any reasonable AEW system will
demand considerable mass storage (disc/drum and magnetic tape) to
maintain tracks in a dense signal environment, while logging histories
on mag tape drives.

USN Grumman E-2C Hawkeye. Israeli Air Force E-2Cs
played a
key role in the crushing defeat of Syria's fighter and SAM forces over
the Bekaa Valley in 1982. Initially these aircraft employed ESM to
pinpoint SAM and radar sites, after these were destroyed in missile and
bomb attacks the E-2Cs coordinated and controlled a series of air
battles which resulted in the loss of over 80 MiGs for no Israeli
losses
in air-air combat.

The limitation imposed by the data processing is in the
number
of tracks which can be maintained before the computer runs out of
horsepower and the system saturates, resulting in software crashes or
tracks disappearing off screens. The failure of GEC's APY-920/Nimrod
was
due in part to problems in this area, with highway traffic incorrectly
tagged as real targets saturating the data processing computer.

C3 and its associated management systems are another key
component of an AEW system, providing voice communications and digital
datalinks to both fighter aircraft and surface defences, the latter of
particular importance, as they can feed target track parameters
directly
into the fire control systems of interceptors and SAM/AAA systems, thus
very tightly coordinating defensive fire. In an environment where
inbound bombers can successfully jam smaller AI radars, the superior
ECCM performance of a sophisticated AEW radar can allow a successful
intercept (the E-2C/F-14/AWG-9 provides for fully automated
intercepts).

The integration of data Processing and C3 systems is thus of
particular importance if a user seeks the full benefit of the AEW
system. Deficiencies in this area could bottleneck the flow of vital
information to the place it is most needed, the fire control system of
a
defending interceptor.

It is readily apparent the functional utility of an AEW
system
depends as much upon the performance of its radar as upon the
capability
of its IFF, ESM, C3, computers and as upon the capability of the
software which ties it all together.

Aerodynamic Performance
and the Airframe

While the systems carried on an AEW aircraft largely
determine
the overall system capability, the airframe and its aerodynamic
performance must be well matched to the mission electronics. An
airframe
must have the internal volume to carry substantial fuel, to fit the
large radar transmitter chain(s), associated power supplies, the
receiver, IFF, ESM electronics racks, the computers and associated mass
storage devices and the opera tor consoles, while providing inflight
maintenance access to repairable critical systems, adequate space for
active crew stations and relief crew accommodation where applicable.

Aircrew accommodation is particularly important, as fatigue
will rapidly degrade operator performance. If missions are longer than
of two to three hours duration, operators will have to be rotated for
breaks. Inflight maintenance access to electronics racks must also be
considered under these circumstances, as it could prevent a mission
abort in the event of an inflight repairable failure not covered by
redundancy. Accessibility in general helps to reduce down-time and
should never be overlooked. Another important aspect is the capacity of
the onboard electrical power generators to supply the vast amounts of
power required for the transmitters, while the engines are at
cruise/endurance thrust settings. Higher thrust settings penalise
endurance. Powerplant loading must also consider power required for
cooling the electronics and transmitter tubes, this can be substantial
in itself. The generators are ideally redundant and may have to be
separately allocated to the transmitter power supply and remaining
electronics supplies, to ensure that the computers and receivers get
clean electrical power without interference leaking through (thus
preventing software crashes or other problems).

Powerplant loading thus will affect aerodynamic performance,
which alone is a key factor in determining the system's capability. The
most important aerodynamic performance parameters are altitude on
station and of course endurance. Altitude on sta tion determines the
distance to the radar horizon and thus the limit upon the detection
range for low level targets, otherwise given by the performance of the
radar, in particular its signal processing. Understandably detection
range for targets above the horizon is greater and determined much by
the performance of the receivers. On station altitude is constrained by
flat (skidding) turn performance and minimum powerplant fuel
consumption
(to maximise endurance), for a turbojet/turbofan near the tropopause
(cca 35,0000 ft) and for a turboprop usually between 20,000 to 30,000
ft
(props require increasing power with altitude, which counters the
improvement in fuel consumption with altitude). Where look-down
detection range is a priority a jet powered aircraft has an advantage,
as is apparent from a mission profile.

A typical mission profile will involve a climb and then
cruise
(cruise climb if a jet) to station, where the aircraft enters a
racetrack pattern which it maintains typically until its allocated fuel
load is consumed, after which it will cruise back to its operating
base.
Inflight refuelling offers a large payoff where the distance to station
exceeds a hundred or so nautical miles, as the fixed overheads in fuel
and time incurred during a return/refuel/turnaround are reduced,
typically by 50% where inflight refuelling has doubled time on station.
This will reduce the number of AEW aircraft required to provide
continuous coverage on a given station.

An AEW aircraft on station will usually control two or more
standing Combat Air Patrols (CAP) in racetrack orbits to the left and
right of the threat axis (an imaginary line between the defended target
and inbound threat) with the risk to the AEW aircraft determining how
far toward the threat the AEW station will be.

Where the enemy uses high performance fighters with long
range
radar and AAM/ARMs, the AEW aircraft will have to be stationed well
behind the CAPs which reduces warning time and the width of the
interception barrier (an imaginary line perpendicular to the threat
axis, through which no hostile should pass). Where AEW detection range
is poor, risk must be traded against the need to stop low level
penetrators firing standoff Precision Guided Munitions, both of which
are the most difficult class of targets to detect and consequently
requiring as forward an AEW station position as possible to maximise
the
warning time.

If CAP is unavailable and ground alert interceptors are used,
the interceptor's dash to station time delay must be accounted for and
the AEW aircraft stationed closer to the defended target, with an
inevitable reduction in the interception barrier width (here again
tankers prove their worth by allowing extended duration fighter CAP).

It is very clear therefore that the choice of AEW radar and
airframe will have to be carefully considered against the various
operational scenarios likely to be encountered, these factors
illustrating the constraints which have led various air forces to
develop very different airframes and radars for the role.

Part II, The Australian Perspective will be presented in the
May edition of Australian Aviation.

PART
II - The Australian Perspective

Australia is in the somewhat
unique position of having to defend a very large, but sparsely
populated
land mass against seaborne and aerial intruders.

These may range from high level threats such as AV-MF and VVS
Bears and Backfires, staging from Cam Ranh Bay and flying peacetime
surveillance/recce and wartime strike missions, through Indonesia's
F-16
and A-4 tactical jets which could be expected to play a major role in
any serious regional dispute, down to general aviation aircraft used to
smuggle drugs into the country or other items (eg; wildlife) out of the
country. While these winged intruders constitute a potential and real
problem, one cannot overlook the environmental threat posed by
Indonesian fishing vessels often caught plundering the rich fishing
areas of the North and North-West, this activity is pursued with
obvious
contempt for Australian quarantine procedures.

Clearly the problems of illegal fishing, immigration and drug
running are the most pressing at this instant in time, the potential
wartime threat of air strikes and supporting recce however, must not be
dismissed, given the rapid economic growth in the North and North-West.

It is fair to say, that the fighting in the Persian Gulf and
recent disasters in the North Sea, graphically illustrate the
vulnerability of onshore and offshore oil/gas installations to
Anti-Shipping Missile (ASM) attack. The Barrow Island area and Withnell
Bay facilities and the North Rankin Platform, are thus lucrative
targets
for a serious opponent. It is not unreasonable to surmise, that the
threat alone of such attack (potentially stopping 30% of our national
natural gas production capacity) could serve to tie many of the RAN's
area defence SAM firing vessels to air defence picket duty covering
these sites, thus effectively reducing RAN resources available for
offensive and other defensive operations.

This situation will be exacerbated if the Timor Sea oil/gas
wells are developed for production. To these targets we must add the
expanding RAAF and Army infrastructure in the Northern Territory and
North of WA and of course Cape York. Were the proposed spaceport to go
ahead, it would represent a target of considerable propaganda value.

To defend the North and North-West the RAAF is expanding a
chain of airfields, some of which are established and several of which
are new, including the hub of Northern operations, Tindal near
Katherine
in the NT. To the West Learmonth, Port Hedland and Derby, to the East
Gove, Weipa and Cairns, these airfields would support detachments of
F-18 aircraft from the squadron home based at Tindal. These aircraft
would receive inflight refuelling support from tankers operating or
staging from Darwin/Tindal and thus, could carry out intercepts several
hundred nautical miles off the coast if necessary, alternately Combat
Air Patrol endurance could be stretched considerably.

Limitations in numbers of aircrew and support personnel would
however dictate ground alert rather than standing CAP which will impose
a need for good early warning of inbound hostiles. While the basing of
interceptors close to targets is an important aspect of air defence, it
is of limited value without extensive low level radar coverage, command
posts and associated C3 facilities.

Long range early warning of inbound threats will be provided
by the Jindalee Over The Horizon Radar (OTHR). OTHR is invaluable as it
can detect and track low flying aircraft and ships at ranges beyond
1500nm. It has limitations though, as its accuracy in range and azimuth
locate a target only within an area of several nautical miles with no
height finding capability, with performance fluctuating subject to
ionospheric conditions.

OTHR will thus provide an indication of an inbound threat and
its location with an accuracy that may or may not provide for a
successful intercept. As it lacks the ability to identify an aircraft
from IFF/ESM/flight profile, its usefulness for the close control of
interceptors is limited, more so since an extensive network of
communications relays would be required to control air defence aircraft
(a HF datalink is another alternative, penalised in performance however
by ionospheric conditions).

These gaps in capability are filled with an AEW aircraft,
which carries ESM, IFF and a centralised C3 network with it, regardless
of the state of the ground based infrastructure it is defending. The
capabilities of AEW and OTHR are thus clearly complementary, with OTHR
providing up to several hours of warning, AEW aircraft and interceptors
can be positioned to provide the best pos sible coverage of targets
accessible by the inbound hostile(s).

The AEW aircraft would then identify the threat and control
intercepts taking advantage of its on board systems. It must be stated
that proponents of either system who reject the alternative have not
understood the difficulties involved in covering the vast area of the
North and North-West with a handful of fighter aircraft.

RAF BAe Nimrod AEW.3. The AEW Nimrod was an
innovative
British design which ran into development problems and in 1986 was
cancelled in favour of the larger Boeing E-3/Westinghouse APY-2 AWACS
system. Many of the Nimrod's troubles stemmed from the choice of an
airframe too small to support the space, cooling and power requirements
of a high performance AEW system resulting in major shortcomings in
range/ endurance. This was compounded by problems in antenna / receiver
/ radome performance and data processing capability. Lower images show
the operator console design, the original cassegrain antenna, and the
replacement offset paraboloid antenna design (GEC Marconi images).

It is expected that the AEW aircraft would be covering the
two
most sensitive areas in the North and North West, operating from
Learmonth and Tindal, with other deployments possibly made to support
operations in other areas if necessary.

The air defence of the North and North West is uniquely
characterised by a low density of possibly very sophisticated threats
operating over land and sea, distances imposing the need for
considerable range/endurance and an undisputable requirement for
extensive C3 tied into a larger national air defence system. These C3
systems should have the ability to feed OTHR target track data to AEW
aircraft for association with ESM, IFF and radar tracks while providing
for the transfer of near real time tactical situation data into the
national air defence system.

The RAAF's eventual choice of AEW aircraft will thus have to
not only account for airframe and radar/IFF/ESM performance, but also
consider the capacity of the airframe and system hardware and software,
to accommodate the hardware and software required for a tie in to what
should eventually become the integrated National Air Defence and
Airspace Control System.